The Centre for Environment, Fisheries and Aquaculture Science (Cefas) is an Executive Agency of the Department of Environment, Food and Rural Affairs (Defra), formerly the Ministry of Agriculture, Fisheries and Food (MAFF). It was also known previously as the Directorate of Fisheries Research (DFR). This data policy refers to data collected by the organisation under all titles.

These data have no specific confidentiality restrictions for academic users. However data are restricted for commercial requests and clearance must be obtained by BODC from Cefas before they are released.

Users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgement.

The recommended acknowledgement is: "This study uses data from the Centre for Environment, Fisheries and Aquaculture Science (Cefas), provided by the British Oceanographic Data Centre."

Guildline 8770 CTD profiling system and 8709 probe

The Guildline 8770 is a portable ocean profiling system comprising a probe (model 8709), control unit and winch. Three versions of the probe are available: standard, Artic and batfish. The differences between the versions lie in numbers and types of connectors, and the existence of a protective cage (not present in the batfish version). The probe has a nominal drop rate of 1 ms -1 and a pressure rating of 1000 dbar.

The probe includes a resistance thermometer temperature sensor, a strain gauge pressure sensor, a conductivity cell, a temperature-compensated polarographic dissolved oxygen sensor and a combination pH sensor incorporating a glass measuring electrode and a calomel reference electrode. The probe digitises all measurements, and phase encodes and transmits the data to the control unit.

Instrumentation

The instrument used was a Guildline Model 8770 CTD. The system consisted of a probe and control unit which measured (or calculated) pressure, conductivity ratio, temperature and salinity. The conductivity sensor is an electrode mounted in pyrex glass, the pressure sensor is a strain gauge element in a four arm resistance bridge network and the temperature sensor is a copper resistance element in a stainless steel tube. The probe electronically measures the parameters and sends the data to the control unit after digitising the results.

The control unit converts the data to binary format, displays the results, has analog output for chart recording and allows the data to be recorded on audio cassette. This latter facility is usually a precaution against information loss since data are logged (at 1 sample per second) on either a Hewlett Packard mini computer or an Apricot microcomputer. Salinity is calculated by a microprocessor within the system, the algorithm used is that for the Practical Salinity Scale.

The CTD is mounted on a General Oceanics multisampler fitted with Niskin bottles. This enables the data to be logged whilst the water sample is being collected. Attaching thermometers to the Niskin bottles allows comparison to be made between reversing thermometers and the CTD temperature sensor. An altimeter has been fitted to the multisampler, this allows a comparison to be made with the pressure calibration.

Calibration

Pressure

The pressure sensor was corrected after calibration with a dead weight tester.

Temperature

Temperature sensor corrected after laboratory calibration with platinum resistance thermometers. The platinum resistance thermometers were set by a triple point cell calibrated at the National Physical Laboratory. Reversing thermometers were used to validate the laboratory calibrations.

Salinity

The calculated salinity was corrected after comparison with the salinity of water samples collected during the cruise. In addition, a precision decade box was used to check the conductivity sensor.

Data Processing

Data are logged every one second. Since the lowering rate is usually less than one metre per second, values are often recorded at between 0.2 and 0.4 decibar intervals. These data have not been averaged, however they have been plotted to identify spurious readings. Estimates of density were also used to help identify suspect values, but in view of the limited accuracy of the instrument and because the water column is well mixed it is inevitable that some density inversions will exist in the data.